Neutral exhaust studies in detached Wendelstein 7-X discharges using EMC3-Eirene

In order to maintain density control in stellarators, it is key to improve our understanding about the neutral exhaust in present experiments. Therefore, hydrogen experiments performed at Wendelstein 7-X are analyzed. In the studied experiments, the plasma density and radiated power fraction are increased during the discharge resulting in a scan from attached to detached plasma conditions. EMC3-EIRENE is a useful tool to analyze the behavior of neutrals as it allows to quantify the number of generated neutrals as well as the number of neutrals leaving the divertor region. In this contribution, experimental data are added to EMC3-EIRENE simulations to improve the overall understanding of the neutrals in Wendelstein 7-X. The infrared camera and the divertor Langmuir probes are compared to the simulated divertor conditions and are used in that way to verify the number of generated neutrals. The Ha camera is used to quantify the ionization of the neutrals which takes place in front of the divertor targets. The generated neutrals which are not ionizing, leave the divertor area. The goal is to increase the fraction of these neutrals which reaches the pumping gap and makes it in that way to the subdivertor region and the pumps. ... mehrThe coherence imaging spectroscopy diagnostic measures the line-of-sight averaged neutral velocity, providing information on the net velocity distribution around the pumping gap. Despite the limited amount, the available experimental data are used to verify the EMC3-EIRENE output, in order to test to what extent the simulations returns reliable predictions. EMC3-EIRENE can write out precisely the generated neutrals and the number which reaches the pumping gap. This shows that, while going
from attachment to detachment, first the number of generated neutrals starts to decrease and only afterwards the number of neutrals making it to the pumping gap is going down.